In a diesel or Compression Ignition (CI) engine air, normally diluted by a small controlled fraction of residual gas, is compressed trough a volume ratio from approximately 12 to 20, and liquid fuel is sprayed into the cylinder during the compression stroke near the top dead center position of the piston (TDC).
Since both the pressure and temperature of the cylinder contents at the time of injection are very high, chemical reactions begin as soon as the first droplets of injected fuel enter the cylinder.
FIG. 1 shows the main parts by which the combustion process is accomplished in a modern CI-engine. The fuel is transferred from the tank (1) through an appropriate filter (2) to a high-pressure pump (3), which delivers the fuel at a pressure between 130 and 200 MPa to a rail (4) common for all the fuel injectors (6a to d). An Electronic Control Unit (ECU) (7), which gathers information of engine speed, temperature, fuel pressure (5) and load target, adapts the engine control parameters to optimize the number of injections and their duration to fulfill both load target and exhaust gas pollution requirements. The injector atomizers are designed to produce a spray pattern (8), which is adapted individually to the combustion chamber geometry (9).
However, in a CI-engine the chemical reactions start so slowly that the usual manifestations of combustion, such as a measurable pressure rise, occur only after the expiration of an appreciable period of time called the delay period. The sum of the injection and the delay periods characterizes the first phase of combustion. The delay period is followed by a pressure rise, which is conditioned by the fuel used, the total quantity of fuel injected with respect to the air trapped in the cylinder (Air-Fuel ratio A/F), the number of injections on which the total amount of fuel is distributed and the Crank Angle (CA) values at which the injections are performed. The pressure-rise period characterizes the second phase of combustion.
The third phase of combustion, called phase 3, starts after the maximum combustion pressure is reached. This blow-down phase will determine the nature and volume of the different post combustion products in the exhaust gas (NOx, Particulate matter, Aldehydes, etc.) and is equally heavily influenced by an appropriate multiple injection strategy.
FIG. 2 shows a typical generic pressure-CA diagram for a diesel engine in which only one single injection is performed in the period between 40° and 20° CA before Top Dead Center (TDC) of the compression stroke. The dashed line represents compression and expansion of air only, without combustion. The continuous line represents compression and expansion with combustion. The injection period is followed by the delay period and their sum equals the phase one.
The main combustion takes place during the pressure rise, called phase two, which terminates at the maximum combustion pressure. For a given fixed definition of the A/F-ratio, the injection strategy, the combustion chamber geometry and the fuel composition the CA-lengths of phase one and two as well as the Pmax-value (slope of pressure rise) are parameters that have a cycle-to-cycle variation of less than +/−3% at a given load point.
The phase three (Blow-down) will by the combustion chamber temperature distribution (absolute level and homogeneity) largely influence the production of eventually unwanted post-combustion products in the exhaust gas.
It is important to understand that the complete pressure-CA diagram together with the induced exhaust gas temperature represent a unilateral signature of both the complete chemical and thermodynamic combustion process (pressure-CA diagram) and the potential equilibrium of pollutant matter in the exhaust gas (Temperature) for a given set of fixed boundary conditions (engine speed, load, injection strategy, overall engine temperature, well-defined standard fuel composition).
The important characteristics of a commercial typical diesel fuel (average chemical formula C12H23) are the ignition quality, the density, the heat of combustion, the volatility (phase one and two as well as Pmax), the cleanliness and the non-corrosiveness. All but the two last properties are completely interrelated. This is why the combustion-quality for a commercial diesel fuel is rated by the Cetane number. As in the case of octane rating of gasoline, diesel fuels are rated with respect to combustion quality by a method that uses engine-test comparisons with reference fuels (e.g. American Society for Testing Materials (ASTM) Standard D613).
The primary reference fuels are normal cetane (C16H34), straight chain paraffin having excellent ignition quality, and alpha-methylnaphthalene (C10H7CH3), a naphthenic compound having very poor ignition quality. A special engine with a compression-ignition cylinder is used as standard equipment for this type of test.
The percentage of cetane in a blend of the above indicated reference fuels giving the same ignition delay as the fuel under test is taken as the cetane number of the fuel in test. As the pressure-CA diagram is a unilateral signature of the combustion process the cetane number is a unilateral signature of the fuel combustion-quality.
The important consequence is that if all engine parameters are kept constant and a fuel with a different cetane number is used, the pressure-CA diagram signature will change as the phase one, two and the Pmax-value changes.
In recent years the presence of bio-fuel blends for SI-engines (mixing of pure gasoline and ethanol at various fractions—flex fuel) have become popular as a very efficient and practical means to decrease the amount of permanently stored CO2 in the atmosphere.
It therefore was suggested to mix current diesel fuel with a fraction of FAME (Fatty Acid Methyl Esters) vegetal-based oil. The higher the percentage of FAME-oil the more important the decrease of the amount of permanently added CO2 in the atmosphere. A mixture containing “x” % of FAME oil and (100-x) % of fossil oil will be referred to as a “Bx” mixture.
For current commercial diesel engines a FAME-oil fraction of less than 20% is acceptable without major changes in the common rail (CR) based injection strategy. Unfortunately at fractions between 20 and 100% the reaction of the combustion process becomes uncontrollable with combustion patterns, which gradually features extreme detonation conditions. An immediate consequence is an important increase in both specific fuel consumption and in exhaust gas pollutants and it can eventually lead to a total misfiring and in extreme cases to the destruction of the engine.
As a matter of fact the pure commercial diesel fuel has an average cetane number of approximately 42, whereas the cetane number of a 100% Fame-oil is typically around 60. The cetane number of a 20% FAME-oil fraction will be approximately 48 to 49, which explains why above this percentage the combustion becomes uncontrollable and action is necessary.